Bugaboo Bee - nest vs footmuff

My son is now four months old, and really too big for the newborn nest that we use in his bugaboo bee. But, the footmuff looks huge! I can't seem to find any recommendations on what sort of age / weight you should move your baby from the nest to the footmuff. Any thoughts?

Don't really know I'm afraid Delbird, but FWIW, we had DD (11 weeks) in the main pram over the hot weather, as the nest was just too much. No footmuff though, and to be fair she is large (probably closer to average size four month old).